WZ theory

In mathematics, specifically combinatorics, a Wilf–Zeilberger pair, or WZ pair, is a pair of functions that can be used to certify certain combinatorial identities. WZ pairs are named after Herbert S. Wilf and Doron Zeilberger, and are instrumental in the evaluation of many sums involving binomial coefficients, factorials, and in general any hypergeometric series. A function's WZ counterpart may be used to find an equivalent and much simpler sum. Although finding WZ pairs by hand is impractical in most cases, Gosper's algorithm provides a sure method to find a function's WZ counterpart, and can be implemented in a symbolic manipulation program.

Two functions F and G form a WZ pair if and only if the following two conditions hold:

Together, these conditions ensure that

because the function G telescopes:

Therefore,

that is

The constant does not depend on n. Its value can be found by substituting n = n₀ for a particular n₀.

If F and G form a WZ pair, then they satisfy the relation

where ${\displaystyle R(n,k)}$ is a rational function of n and k and is called the WZ proof certificate.

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